Windshear detection system

ABSTRACT

A system for processing signals representative of airspeed error and inertial longitudinal acceleration of an aircraft to provide an output signal representative of windshear induced acceleration which is filtered to remove the high frequency components and which may be utilized to indicate the presence of windshear to a pilot who can then respond by resetting the thrust to maintain airspeed, or alternatively the output signal is utilized to correct the inertial damping signal of an airspeed control system under windshear conditions.

This is a division of application Ser. No. 416,540, filed Nov. 16, 1973and now U.S. Pat. No. 3,892,374, which in turn is a division ofapplication Ser. No. 342,917 filed Mar. 19, 1973 and now U.S. Pat. No.3,840,200.

This invention relates to throttle control systems for aircraft and moreparticularly to a throttle control system providing reduced throttleactivity in turbulence.

In prior art speed controls, efforts have been made to compensate forthe influences of turbulence, e.g., in U.S. Pat. No. 3,448,948 toREERINK, however only partial turbulence compensation has been achievedsince the phenomenon and its effects do not appear to have been fullyrecognized or appreciated.

Accordingly, it is an object of this invention to provide a speedcontroller for aircraft wherein the influence of turbulence issuppressed and also windshear is detected and the influence thereof alsosuppressed.

In accordance with the present invention, it is recognized that aprimary effect of a longitudinal gust hitting the aircraft is dragchange proportional to the speed change, causing an acceleration signalwith 180° phase difference relative to the speed change. A secondaryeffect is caused by the response of the parameters pitch attitude (θ)and angle of attack (α) to the same gusts and results in an air speederror signal and longitudinal acceleration signals which have a 90°phase difference. A third effect occurs because the gust generally doesnot strike along the longitudinal axis, but has also a perpendicularcomponent which directly affects α, which in turn results in air speederror and longitudinal acceleration signals which have 90° phasedifference.

The phase difference between the resulting air speed error andlongitudinal acceleration signals induced by turbulence is thereforebetween 90° and 180° and is frequency dependent. In the high frequencyrange where the aircraft does not respond the phase angle approaches180°, however secondary responses of the aircraft to the gust aredominant in the lower frequency range resulting in phase anglesapproaching 90°.

Having recognized the above complex phase relationship between the airspeed error and longitudinal acceleration signals induced by turbulenceit is a further object of this invention to provide throttle immunity toturbulence without compromising other performance aspects of thethrottle control system.

The above and other objects of the present invention are achieved inaccordance with a preferred embodiment by processing signalsrepresentative of air speed error and longitudinal acceleration. Thephase lead of the acceleration signal may be increased relative to theair speed error signal to 180°, or, the air speed error signal may belagged relative to the longitudinal acceleration signal to achieve the180° phase difference relationship, or a combinatioan of lead and lagnetworks may be utilized in accordance with the preferred embodiment ofthe invention as hereinafter described.

Further features and advantages of the invention will be apparent withreference to the specification and drawing wherein:

FIG. 1 is a simplified block diagram deemed helpful in understanding howthe throttle command signal (δ_(T).sbsb.c.sbsb.m.sbsb.d) is developed inthe present throttle control system;

FIG. 2 is a block diagram similar to the block diagram of FIG. 1 howeverfurther including means for improving the servo response of the systemof FIG. 1;

FIG. 3 is a block diagram of a circuit for isolating the windshearcorrection term (ΔV) utilized in the present autohrottle control system;

FIG. 4 is a block diagram of a windshear detector circuit utilized inthe present throttle control system and which is similar to the circuitof FIG. 5 and includes a rate limited lag circuit for filtering out ofthe signal representative of the windshear correction term (ΔV) the highfrequency components of the rate of change of air speed due to randomatmospheric turbulence; and,

FIG. 5 is a block diagram of the preferred embodiment of the presentthrottle control system utilizing the shear detector circuit of FIG. 4for providing shear compensation.

Turning now to FIG. 1 which is a simplified block diagram of the presentthrottle control system, it can be determined that the basic throttlerate command signal (δ_(T).sbsb.c.sbsb.m.sbsb.d) is equal to K₁ V_(E) +K₂ V_(b) where K₁ and K₂ are constants and V_(E) is a signalrepresentative of air speed error relative to selected control speedwhile V_(b) is a signal representative of inertial longitudinalacceleration of the aircraft. Then: ##EQU1##

This throttle rate command signal causes the throttles to move smoothlyuntil: ##EQU2##

The system will minimize E so that equation (1) is closely satisfiedresulting in a decay of the signal representative of air speed error sothat the exact solution to equation (1) is approached whereby ##EQU3##where ##EQU4## and equation (2) then is representative of the desiredideal response of the system, E being a measure of the deviation fromthis ideal response.

The value of the gain of amplifier means K₁ determines the amount ofdeviation from the ideal response (overall tightness with which equation1 is controlled), and ##EQU5## determines the air speed error decay timeconstant.

When the gains of amplifier means K₁ and K₂ are selected in the presentrate command type system to give the same loop gains as in the knowndisplacement type system, the present system provides improved throttleresponse and captures selected air speeds with minimal overshoots. Thedynamic response characteristics of the engine and aircraft are theprimary factors in determining the amplifier gains K₁ and K₂ which yieldthe desired speed control and speed control stability in the system ofFIG. 1.

For large values of τ₁, the amplifier gain K₁ can be selected to satisfyboth air speed and throttle requirements. For decreasing values of τ₁,the value of amplifier gain K₁ must increase to satisfy equation (1),and the basic control law as expressed by equation (1) for thesimplified system of FIG. 1 still provides satisfactory air speedresponse. However, for decreasing τ₁, throttle servo response to a stepchange in selected air speed in the system of FIG. 1 becomesunderdamped. For a preferred value of time constant for speed control(τ₁) of 10 to 15 seconds, throttle servo response can be improved by theaddition of a throttle displacement input signal proportional toinertial longitudinal acceleration of the aircraft (V_(b)) in the mannernow shown in FIG. 2 where amplifier means having a gain K₃ is coupledback to the summing means which is directly coupled to the input of thethrottle servo. In the throttle control system of FIG. 2, subsequent toa step change in selected air speed, the throttle servo first respondsby providing a rate proportional to air speed error relative to selectedcontrol speed (V_(E)). The signal representative of inertiallongitudinal acceleration of the aircraft (V_(b)) gradually increases inamplitude and cancels the signal representative of air speed errorrelative to selected control speed (V_(SEL)). V_(b) lags the incrementalthrottle lever position (δ_(T)) inputted to the engine by the engine lagtime thereby causing the throttle to initially overtravel the positionrequired to provide the ideal rate of change called for by the equationK₁ V_(E) + K₂ V_(b) = O The throttle displacement input signalproportional to inertial longitudinal acceleration of the aircraft(V_(b)) is amplified by the gain K₃ to provide a throttle displacementequal and opposite to the aforementioned overtravel. For changes inselected air speed (V_(SEL)), the throttle will now move directly to theposition which gives the commanded rate of change of air speed. Thethrottle control law for the system of FIG. 2 now satisfies therequirements for air speed select capture and tracking, and exhibitssmooth limited throttle travel without overshoot.

The throttle control system shown in the block diagram of FIG. 2 islinear and as a consequence will permit rates of change of air speederror proportional to air speed error itself. For large step commands itbecomes desirable to limit the acceleration and deceleration levels forinsuring passenger comfort and providing limitation of thrusttransients. Accordingly, upon full implementation of the system of FIG.2 as shown in FIG. 5, a limiter circuit 30 is provided in the firstchannel for processing signals representative of air speed error so thatfor air speed error signals greater than a predetermined air speed errorlimit, the control law is satisfied when ##EQU6## When during signalprocessing in the system of FIG. 5, V_(E) decreases below thepredetermined limit the aforementioned linear control referred to inconnection with the system of FIG. 2 is resumed.

The throttle displacement input signal proportional to inertiallongitudinal acceleration of the aircraft (V_(b)) coupled to addercircuit 26 through the circuit path including amplifier means 28 havingthe gain K₃ as shown and previously discussed in connection with thedescription of FIG. 2, can be also seen in the full systemimplementation of FIG. 5. This input signal besides preventing throttleovertravel also provides for smooth capture of the commanded rate ofchange of air speed error in the present system. In the present systemof FIG. 5, the air speed limit provided by limiter circuit means 30 wasselected to give a V_(b) limit of about 1 knot/second.

Proceeding now to the gust filtering characteristics of the presentsystem, it should be first noted that prior autothrottle systems haveutilized a complementary filter having a time constant (τ) of from 2 to5 seconds to reduce the effect of turbulence on throttle activity. Dueto the occurrence of extremely high rates of change combined withrelatively low frequencies of air speed perturbations in turbulence,such a complementary filter does not adequately attenuate nuisancethrottle activity. Also for increasing values of gust filter timeconstant (τ), the autothrottle coupling with the path control autopilothas a tendency to deteriorate. The basic aircraft response to alongitudinal air mass velocity disturbance may be represented by thefollowing equations: ##EQU7## where τ₂ is the speed response timeconstant due to a thrust-drag difference ##EQU8## assuming the aircraftis speed stable. Therefore the primary throttle response in turbulencedue to the combined signals K₁ V_(E) + K₂ V_(b) becomes ##EQU9## andthis can be reduced to zero by selecting τ₁ = τ₂. Practically, this ispossible only where the natural speed response time constant τ₂ has avalue which is also acceptable for the controlled speed response timeconstant (τ₁). However, a substantial amount of secondary throttleactivity would still occur due to excitation of pitch attitude θ andangle of attack α which in turn induce accelerations and air speederrors having a relative phase difference of 90°. As a result, the phasedifference between the total acceleration and air speed error signalvaries from 180° to 90° depending on frequency.

Total signal cancellation is possible only when the relative phase angleis 180°. This is accompolished by a combination of lead/lag circuis inthe acceleration signal path and air speed error signal path. First, theproportional displacement term K₃ V was found to improve both throttleservo damping and throttle response to the lower range of turbulencefrequencies by providing extra lead in the throttle command due to theacceleration signal V, thereby approaching a phase angle of 180° betweenthe individual throttle commands due to air speed error andacceleration.

To avoid accelerometer noise reaching the throttle command, thisthrottle command signal proportional to acceleration is filtered by thesmall lag provided by filter 101 resulting in a throttle positioncommand due to V_(b) of: ##EQU10## Additional phase correction isobtained by introducing a small lag in the air speed error signalchannel resulting in a throttle position command due to air speed errorof: ##EQU11## These small lags provided by filter 103 and 101, do nothave appreciable influence on the normal system dynamics. The requiredcharacteristics of lag filter 103 to bring about the desired 180° phasedifference between throttle commands resulting from turbulence inducedair speed error and longitudinal acceleration are determined bycomputing the individual open loop throttle command Bode plots due toeach of these signals. From these two Bode plots, the requird air speederror lag filter 103 was constructed. Using this technique it was foundthat a 0.5 second lag in he signal V_(E) representative of air speederror together with optimization of the relative gains (K₁, K₂ and K₃)produced excellent turbulence cancellation for frequencies higher than1.0 rad./sec. In the frequency range above 1.0 radians per second,turbulence cancellation can be improved by insertion of 0.8 seconds lagfilter 105 in the V_(b) signal channel.

The equation for the total autothrottle control law of the system ofFIG. 5 can now be given with the above mentioned V_(E) and V lagsincluded and is ##EQU12##

The lack of throttle response what may presently be termed a "tuned"control law to velocity disturbances in the air mass would howeverresult in low quality performance in windshear, and therefore thepreviously discussed control law representative of the system of FIG. 5but without the circuits 58 and 60 is inadequate in terms of shearperformance. The following immediate description relates to the problemof air speed error control in sustained windshear and development of acircuit comprising a shear detector having low turbulence sensitivitywhich may be utilized in the present autothrottle control system of FIG.5.

Air speed errors due to windshear can be made the overriding controlinput by washing out the V_(b) signal and complementing it with derivedrate to make up for the loss of V_(b) signal and the performance inwindshear can be made satisfactory for values of τ less than 10 seconds,but the derived air speed rate reintroduces the throttle activityproblem in turbulence and therefore this approach is unsatisfactory.Even the utilization of a linear second order complementary filter insuch an approach cannot provide sufficient turbulence suppression whileallowing adequate shear performance. A further disadvantage of such anapproach utilizing linear filters is the requirement of a separate speedinput signal to complement the washed out signal V_(b). Use of thesignal V_(E) would result in throttle response having excessiveovershoots for step changes in the commanded air speed due to ratetaking of the step input. The use of additional in-series-filtering onthe signal V would degrade air speed tracking performance which cannotbe tolerated. As a consequence of the preceding, the circuit 58 of FIG.3 was conceived to actually detect a sustained windshear and correct thecontrol law of the present autothrottle control system by providing awindshear correction signal component (Δ ^(v)) for subtracting from thesignal representative of longitudinal acceleration of the aircraft(V_(b)) to provide a signal representative of longitudinal accelerationof the aircraft compensated for windshear (V). In this manner the airspeed select and tracking performance are preserved, however the signalΔV has a substantial turbulence noise content.

In FIG. 3: ##EQU13## When there is no turbulence or windshear V_(b) =SV_(E) and therefore V = V_(b).

Under windshear conditions, that portion of V_(b) which does not cancelagainst the derived rate of change of air speed error, will form thewindshear correction term Δ V for compensating the signal V_(b) forwindshear. In the circuit of FIG. 3, the windshear correction signal ΔVis isolated and provided in a single signal path as an input to theadder in the V_(b) signal channel, and as a consequence may be filteredas shown in FIG. 4 to permit only the desired low frequency windshearcomponent to be added. In the design of the present shear detectorcircuit, turbulence and windshear are distinguished on the basis offrequency differences and their effect on the controllability.

The controllability of a wind disturbance by an autothrottle systemdepends upon the engines thrust response to throttle lever changes andthe aircraft's speed response to thrust changes. The engine may berepresented by a 1 to 2 second first order lag and the aircraft with atime constant of 10 to 15 seconds. Efficient control of winddisturbances, that is, without excessive throttle activity, is thereforepossible only for frequencies lower than 1/15 radian per second or about0.01 cycles per second. Additional lags in the autothrottle control lawfurther degrade the controllability of atmospheric wind disturbances.Frequency response plots of the transfer function ##EQU14## where Δδ_(T)is the change of throttle lever position in a known autothrottle systemshowed that speed errors V_(E) and increase due to the operation of theautothrottle system for frequencies of ΔV wind higher than about 0.01cycles per second. Therefore in the present control law of FIG. 5throttle response to atmospheric wind disturbances above 0.01 cycles persecond was minimized. A further distinction between turbulence andwindshear is found in the rate of change of air speed V_(E) . Inwindshear, on approach, the maximum V_(E) due to windshear is about 1knot per second. However, in turbulence V_(E) goes to several orders ofmagnitude higher for short periods to time. These high values of V_(E)in turbulence which would cause the undesired throttle activity can befiltered out entirely as seen in FIG. 4 by the rated limited lag circuit60. The time delay on the windshear correction signal ΔV permitted islimited by the requirement for adequate windshear performance. Thelowest possible rate limit is therefore a function of the time constantof washout and lag circuit means 58. Since the rate limited lag circuitmeans 60 is more effective than washout and lag circuit means 58 inattenuating response to turbulence, adequate shear performance withmaximum turbulence attenuation is therefore obtained by a combination ofa small washout and lag circuit 58 time constant and a low rate limit incircuit 60. This small time constant for washout and lag circuit 58 isalso desired for minimizing the undesired ΔV signal in case of a stepchange in V_(E) caused by speed command input V_(SEL) (see FIG. 5)changing V_(E) in FIGS. 4 and 5. With a small time constant for circuitmeans 58, the shear correction term ΔV is detected without appreciabledelay and the windshear performance is then substantially dependent uponthe loop gain K₅ and the rate limit of circuit 60.

Further considerations in the design of the present shear detectorcircuit already described having low sensitivity to turbulencecomprising circuits 58 and 60 for providing the correction term ΔV to besubtracted from V_(b) are now noted. It was noted earlier that a singlecomplementary washout and lag type filter having a time constant of 10seconds provided adequate windshear performance. This allows ΔV to bebuilt up with a rate of 0.1 knot per second² for a step input of 1 knotper second. The rate limit for limiter circuit 50 in the present ratelimited lag circuit 60 may therefore be set at 0.1 knot per second² fora loop gain K₅ of 0.1. A further consideration affects the selection ofthe values of gains K₄, K₅, of amplifier circuit means 46 and 52respectively and the rate limit of limiter circuit 50 in the system ofFIG. 5. The smaller the rate limit selected, and the higher gain valueK₄ that is chosen, the higher percentage of time the rate limit circuit50 will be saturated by turbulence thus preventing the development bycircuits 58 and 60 of a signal ΔV to be subtracted from the signal V_(b)to provide a signal V representative of longitudinal acceleration whichis corrected for windshear. Shear detection and compensation as providedby circuits 58 and 60 coupled between the V_(E) signal channel and theV_(b) signal channels would in such a case be adversely affected by thelevel of turbulence. This is minimized by the present system design byselecting K₄ = 5, K₅ = 0.2 and a rate limit of 0.2 knots per second².These values sufficiently suppress turbulence response of the presentshear detector circuit and do not deteriorate the turbulence immunity ofthe present autothrottle control system. The present shear detectorutilizes as an input, the air speed error signal V_(E) without affectingautothrottle system performance for step changes in air speed. For astep introduction of a 1.0 knot per second windhsear windshear smoothair, the peak value of air speed error remains limited to about 4 knots.

Turning now to FIG. 5 a further advantageous feature of the present ratecommand type autothrottle system will be noted in the mode of operationoccurring when either the forward or aft throttle limit position isreached. When either of these two autothrottle conditions is detected bythe closing of one of throttle limit switches 70 or 72, autothrottlelimit logic circuit 74 generates at the output thereof a servo loopdisengage signal causing switching means 76 to close a signal pathincluding synchronizing amplifier 78 from the output of adder 26 back tothe input of adder 19 thereby synchronizing the total servo commandinput to servo means 10 to zero. The present autothrottle control systemis reengaged subsequently when the sum K₁ V_(E) + K₂ V changes sign(polarity from zero). Sign detector circuit 80 or 82 detect the positiveor negative polarity change respectively of this sum as provided at theoutput of adder circuit 18. This circuitry to provide anticipation ofthrottle command to drive the throttle out of the limit position istherefore proportional to V, as required to provide capture of theselected speed V_(SEL) asymptotically. The total servo position error,(δT_(CMD-)ΔδT) is synchronized to zero when switching circuit 76 is inthe disengaged position to insure that the servo 10 comes out of thelimit position without a step transient. Such a step transient couldoccur due to the presence of the position command signal proportional toacceleration coupled through amplifier 28 and present as an input toadder 26, if this signal was not zeroed by he synchronization loop.Switching means 76 is driven to the engage position when the output ofAND circuit 92 is high, which requires that the system engage switch 90is engaged and both outputs of circuit 301 and 300 are high.

The output of circuit 300 is normally high except when circuit 70 ishigh, signifying that the forward throttle limit is reached and circuit82 is low further signifying that there is no command to drive thethrottles aft, so that in this case both inputs to circuit 300 are highand the output of circuit 300 is lost. The output of circuit 301 isnormally high except when circuit 72 is high signifying that the aftthrottle limit is reached and circuit 80 is low further signifying thatthere is no command to drive forward, so that in this case both input tocircuit 301 are high and the output of circuit 301 is low.

The gain value for K sync amplifier 78 determines how fast the positionerror is nulled out. For a gain factor of 10 the position error goes tozero in less than 1 second.

The feedback loop for the position servo 10 comprises tachometer means84 coupled from the output of servo 10 back to an input of adder circuit19. If the servo motor 10 rotates at a given rate, then he throttleposition δ_(T) is a ramp function. Mathematically the change in throttleposition Δδ_(T) is the integration of the servo or throttle rate, thatis ##EQU15## The tachometer 84 is actually a generator which produces asignal proportional to the angular velocity of the motor 10 orproportional to the differentiated throttle position that is δ_(T) =Sδ_(T). Throttle position as the feedback signal is obtained using thetachometer signal K_(T) Sδ_(T) =K_(T) δ_(T), which is then integrated incircuit 16 yielding ##EQU16## thereby providing an output signalproportional to the actual position change Δδ_(T) utilized to cancel thethrottle position command signal ΔδT_(CMD) at adder circuit 26. Theservo 10 therefore sees a signal outputted from adder 26 which isproportional to the difference of throttle position command provided inthe system of FIG. 5 in accordance with the present autothrottle controllaw and the signal representative of actual throttle position changeΔδT. The throttle servo motor 10 will therefore run with an angularvelocity proportional to the position error of the throttle 94 and cometo a stop only when true position error has reached zero.

The servo motor 10 drives the throttle means 94 through a clutch means96 which is normally engaged. The throttle levers indicative of throttleposition 98 control the amount of fuel passing to engine 99. When thepilot applies a force to the throttle levers denoted ΔδT pilot, clutchmeans 96 disengages so that the throttle servo 10 no longer drives thelevers. This allows the pilot to take over throttle control at any time.

A highlight of the present autothrottle control system is operation withhigh air speed error integral gain and this is made possible because thebasic response characteristics are determined by the relative values ofair speed error and acceleration gains provided in the system. All gainsmay therefore be increased together to satisfy requirements for high airspeed error integral gain required for aircraft configuration changeswithout substantially deteriorating other performance areas of thesystem such as air speed select change performance and response of thesystem in turbulence.

Where it is desired to utilize the system of FIG. 5 in a deceleratingapproach mode, the air speed error signal V_(E) developed as an input tothe present system will be the difference between commanded air speedV_(SEL) and actual air speed; however, V_(SEL) would be inputted as afunction of flap position which in turn is a function of altitude abovethe runway. However, the system of FIG. 5 would in the deceleratingapproach mode of operation require an extra deceleration command signalsummed in the signal path V with this shear corrected V signal, e.g., atadder circuit 56. The decelerating command signal to be summed isderived by differentiating the flap-programmed speed command signal.

Returning now briefly to the shear detector circuit means including theactual detector circuit 58 and correction signal generating circuit 60responsive to circuit 58, it was noted that the present shear detectorcircuit in essence corrects the discrepancy between the air speed errorsensor signal and the accelerometer signal in the low frequency range(e.g., below 0.1 rad/sec) without allowing higher frequency disturbancesof the control reference signal (air speed error V_(E)) to increase thenoise content in subsequent signal processing of these signals. Thepresent shear detector circuit means thus has application in othersystems which require air speed referenced longitudinal acceleration. Itshould be noted that the circuit corrects for all long term errorsincluding, e.g., errors due to accelerometer attitude errors causing theaccelerometer to detect gravity components. Therefore, the circuit maybe further utilized in applications requiring correction of long termdiscrepancies between a pair of control reference and damping signalsensors such as altitude and altitude rate signal sensing means oraltitude rate and normal acceleration sensing signal means.

what is claimed is:
 1. An aircraft instrument comprising first means forproviding a first signal representative of the airspeed deviation of anaircraft from a preselected command speed;second means for providing asecond signal representative of the inertial longitudinal accelerationof the aircraft; third means for processing said first and secondsignals, said third means including washout and lag circuit means forproviding an output signal representative of the difference between thelagged inertial longitudinal acceleration and the lagged rate of changeof airspeed deviation; and fourth means connected to the output of saidthird means, said fourth means comprising a rate limited lag circuit forproviding at the output thereof the low frequency signal component ofsaid output signal of said third means, said low frequency signalcomponent indicative of a windshear condition.
 2. A method for providinga windshear correction signal component of an inertial damping signal inan automatic speed control system of an aircraft, said method comprisingthe steps of:deriving a first signal representative of the lagged rateof change of the deviation of airspeed from a selected command signal;deriving a second signal representative of lagged inertial damping, andfurther subtracting said first signal from said second signal to providesaid windshear correction signal component.
 3. The method of claim 2further including the steps of:filtering said windshear correctionsignal component with a rate limited lag circuit for noise content inthe frequency range higher than the range of desired controlfrequencies.
 4. The method of claim 3 comprising:correcting saidinertial damping signal of said automatic speed control system forwindshear by subtracting the output of said rate limited lag circuitfrom said inertial damping signal.